Pai is literally my favorite Thai restaurant in downtown Toronto. The ambiance is great, Thai style with some nice Toronto touch (raptors, we the north).
You can never go wrong with the food here. My favorite is Pad Gra Prow. It is stir fried minced meat on rice. So tasty. I would go with foreigner spicy. Unless you are really really good with spicy, don't try Thai spicy. I learned my lesson the hard way :P Their chicken wings are very good too. The portion is small though. One order comes with three wings.
The line can be long during lunch and dinner. They take reservation in advance but not on the same day.
They also have a party room behind the bar that is perfect for a small party.